Double clad optical fiber having ring core surrounding core for high power operation

ABSTRACT

An optical fiber apparatus having a wavelength of operation comprises an optical fiber comprising a core; a pump cladding disposed about the core for receiving pump optical energy having a pump wavelength; and a second cladding disposed about for tending to confine pump optical energy to the pump cladding. The core can comprise a rare earth material for providing optical energy having the wavelength of operation responsive to the optical fiber receiving the pump optical energy, and the fiber can further comprise at least one ring core spaced from the core, the ring core defined by inner and outer diameters and comprising the cross sectional area therebetween. The ring core can comprise an absorbing material for absorbing optical energy having the wavelength of operation. At the wavelength of operation the optical fiber can comprise a fundamental mode that is primarily a mode of the core and at least one higher order mode (HOM) that is a mixed mode of a selected mode of the core and of a selected mode of the ring core, the selected modes being of the same azimuthal order. The mixed mode can be suppressed relative to the fundamental mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is continuation of U.S. patent application Ser. No.12/508,223 filed with the US Patent and Trademark office on Oct. 15,2009.

FIELD OF THE INVENTION

The present invention relates to optical fibers and optical fiberapparatus, such as, for example, optical fiber amplifiers, lasers andamplified spontaneous emission (ASE) sources.

BACKGROUND OF THE DISCLOSURE

Lasers, amplifiers and other optical apparatus based on optical fibercan provide flexible, rugged and relatively simple sources of opticalenergy. Accordingly, in many applications such optical fiber apparatuscan often have one or more advantages as compared to counterparts basedon a gas medium (e.g., CO₂) or on a bulk solid-state medium (e.g., aNd:YAG rod). For example, optical fiber lasers can have a smallerfootprint, or can be more efficient, or can require less sophisticatedcooling arrangements as compared to using a gas or bulk rod solid-statelaser in a similar application. Often, however, it can be desirable toincrease the output power of optical fiber apparatus, as certain gas andbulk solid-state lasers can readily produce high CW output powers orpulses of optical energy having high energy and/or high peak power.

Unfortunately, because of the high power density inherent in confiningoptical energy to the relatively small cross sectional area of anoptical fiber, non-linear phenomena, such as Stimulated Raman Scattering(SRS) or Stimulated Brillouin Scattering (SBS), can severely limitscaling the output power of a fiber laser or amplifier to higher powers.Though these non-linear processes are complex, each can be addressed, atleast in part, by reducing the power density in the core of the fiber.One way to reduce power density is to increase the diameter of the coreof the fiber and/or reduce the numerical aperture (NA) of the core, suchthat the fiber has a larger mode field diameter (MFD). Reducing thepower density in this manner can increase the power threshold for theonset of the undesirable non-linear phenomena.

This approach, however, is not without drawbacks. Fibers having largercore diameters can typically support higher order transverse modes(e.g., LP₁₁, LP₂₁, LP₀₂ etc.) in addition to the fundamental mode (e.g.,LP₀₁). Such higher orders modes (HOMs) tend to degrade the quality ofoutput optical energy provided by the fiber apparatus and hence raisethe M² parameter (lower M² means better beam quality). In manyapplications a low M² is desired. Forestalling the onset of non-lineareffects while also maintaining good beam quality can present a challengeto the designer of optical fiber apparatus.

Some approaches to this challenge are known in the art. For example,U.S. Pat. No. 6,496,301, issued on Dec. 17, 2002 to Koplow, Kliner andGoldberg, teaches bending a multimode fiber having a larger core tosubstantially attenuate, via increased bend loss, higher order modessuch that a fiber amplifier provides gain in substantially a singlemode. See also U.S. Pat. No. 7,424,193, issued on Sep. 9, 2008 toAlamantas Galvanauskas, which teaches a composite waveguide having acentral core and at least one side core helically wound about thecentral core and in optical proximity to the central core. According tothe '193 patent, higher order modes of the central core selectivelycouple to the helical side core and experience high loss such that thecentral core is effectively single-mode.

Existing techniques, however, are not necessarily entirely satisfactoryin all circumstances. Accordingly, it is an object of the presentinvention to address one or more of the deficiencies or drawbacks of theprior art.

SUMMARY OF THE DISCLOSURE

In one aspect, the disclosure provides an optical fiber apparatus havinga wavelength of operation at which the optical fiber apparatuspropagates optical energy, where the optical fiber apparatus includes anoptical fiber. The optical fiber can comprise a core; a pump claddingdisposed about the core for receiving pump optical energy having a pumpwavelength; and a second cladding disposed about the pump cladding fortending to confine pump optical energy to the pump cladding forpropagation thereby. The core can comprise a rare earth material forproviding optical energy having the wavelength of operation responsiveto the optical fiber receiving the pump optical energy having the pumpwavelength. The optical fiber can also include at least one ring corespaced from the core, wherein the ring core is defined by inner andouter diameters and comprises the cross sectional area therebetween, andthe ring core further comprises an absorbing material for absorbingoptical energy having the wavelength of operation. The optical fiber canbe configured and arranged such that at the wavelength of operation theoptical fiber comprises a fundamental mode that is primarily a mode ofthe core and at least one higher order mode (HOM) that is a mixed modeof a selected mode of the core and of selected mode of the ring core,wherein the selected mode of the core and the selected mode of the ringcore are of the same azimuthal order. The mixed mode can be suppressedrelative to the fundamental mode.

In additional aspects of the disclosure, the selected mode of the corecan comprise the LP₀₂ mode of the core. The selected mode of the corecan comprise the LP₁₁ mode of the core. The at least one HOM cancomprise an intensity profile having a substantially central maxima or asubstantially central minima. The at least one HOM can comprise at leasttwo HOMs, one of which comprises a substantially central maxima and oneof which comprises a substantially central minima. The pump cladding canconsist or consist essentially of silica-based glass or pure silica. Inone practice, the fiber does not include a microstructure for guidingoptical energy. The rare earth material can comprise at least one ofytterbium, erbium, neodymium or thulium. The absorbing material cancomprise at least one of samarium, praseodymium or terbium. The pumpwavelength can comprise a wavelength of about 790 nm, and the absorbingmaterial can comprise samarium. The pump wavelength can comprise, forexample, a wavelength of about 793, about 915, about 940, about 976, orabout 1567 nm. The pump wavelength can comprise a wavelength of about1567 nm, and the absorbing material can comprise terbium. The wavelengthof operation can comprise, for example, about 1060 nm, about 1550 nm, orabout 2000 nm. In one example, the wavelength of operation can comprisea wavelength of operation of about 1060 nm and the absorbing materialcan comprise samarium or praseodymium. The rare earth material cancomprise neodymium, and the absorbing material can comprisepraseodymium. The at least one HOM can be suppressed such that thepropagation loss of the at least one HOM is at least 10 times, in termsof dB per meter, higher than the propagation loss of the fundamentalmode at the wavelength of operation. The ring core can comprise asilica-based glass. The silica-based glass can comprise a concentrationof one or more of aluminum, fluorine, germanium or phosphorus, such as,for example, a concentration of aluminum and/or germanium.

In further aspects of the disclosure the optical fiber can comprise atleast one longitudinally extending stress inducing region having athermal coefficient of expansion that is different from material of theoptical fiber disposed about the stress inducing region. The at leastone stress inducing region for increase the birefringence of the opticalfiber for providing polarization maintaining propagation of thefundamental mode. The optical fiber can extend along a longitudinalaxis, and the optical fiber apparatus can comprise a secondlongitudinally extending optical fiber, wherein the second optical fiberis located alongside the first optical fiber such that pump opticalenergy propagated by the second optical fiber can couple to the pumpcladding of the optical fiber for optically pumping the active material.A selected cladding can be disposed about the optical fiber and thesecond optical fiber.

In yet other aspects of the disclosure, the optical apparatus can beconfigured as a laser having a laser cavity defined by at least oneoptical fiber Bragg grating. The optical fiber apparatus can be a laserincluding a master oscillator power amplifier (MOPA) arrangement. Themaster oscillator need not comprise a fiber-based device, and can, forexample comprise a semiconductor element, such as, for example, a laserdiode or a solid state laser. The rare earth material can comprisethulium, and the absorbing material can comprise terbium.

According to yet a further aspect of the disclosure there is also taughta method of designing and/or fabricating an optical fiber having a mixedmode, which method can comprise the steps of: selecting a first mode ofthe optical fiber, such as a mode of a region of the optical fiber, thatis to be mixed with another mode to form a mixed mode; determining theazimuthal order and effective refractive index of the selected firstmode; selecting a mode of at least one other region of the fiber to havesubstantially the same effective refractive index and same azimuthalorder as the first mode; and constructing and arranging the design ofthe fiber such the selected modes mix to form a mixed mode.“Substantially the same” in this context means close enough so that theselected modes will mix to form the mixed mode.

In more aspects of the disclosure, the optical fiber comprises at leastone longitudinally extending stress inducing region having a thermalcoefficient of expansion that is different from material of the opticalfiber disposed about the stress inducing region. The stress inducingregion can increase the birefringence of the optical fiber. Thebirefringence can be increased such that optical fiber comprises apolarization maintaining (“PM”) optical fiber. The optical fiber cancomprise a rare earth material for providing optical energy having theoperating wavelength response to the optical apparatus receiving opticalenergy having a pump wavelength. The core of the optical fiber cancomprise a diameter of at least 15 microns, a selected numericalaperture, and a V-number at the operating wavelength of greater than 3.The V-number can be greater than 5. The selected numerical aperture canbe no greater than 0.10, or, alternatively, no less than 0.13 or no lessthan 0.15.

Certain terms used herein are now generally discussed. Others arediscussed in the Specific Description and elsewhere below.

“Primarily a mode of the core” or “primarily a core mode” means that themode (e.g., the fundamental mode) is not a mixed mode of the core andthe ring core, where at least one HOM is a mixed mode of the core andthe ring core spaced therefrom. In other words, the properties of themode that is primarily a mode of the core are substantially determinedby the core properties and the properties of the cladding, with thepresence of the spaced ring core of which the at least one HOM is amixed mode having little effect on the properties of the mode.

“Substantially higher propagation loss,” as that term is used herein,means that the loss, as measured in dB per unit distance (e.g., permeter) is at least five (5) times higher at the wavelength of operation(e.g., at least 1.0 dB/meter if the baseline for comparison is 0.2dB/meter). Such propagation loss can be determined on the basis of atest fiber that does not include a rare earth material, as such materialmay also absorb optical energy at the operating wavelength and may makecomparisons difficult (e.g., the problem of measuring a relatively smalldifference between relatively large numbers). Stating that one mode issuppressed relative to another mode means that it has substantiallyhigher propagation loss than the other mode. It is noted that the terms“index of refraction” and “refractive index” are at times usedinterchangeably herein. “Multimode” means not single mode, and includeswhat is sometimes referred to in the art as “few-moded.” Typically amultimode fiber has a V-number of greater than 2.405 at its operatingwavelength. “Material” includes material in the forms of ions (e.g.,“comprising a concentration of erbium” includes comprising aconcentration of Er3⁺ ions).

Further advantages, novel features, and objects of the invention willbecome apparent from the following detailed description of non-limitingembodiments of the invention when considered in conjunction with theaccompanying FIGURES, which are schematic and which are not necessarilydrawn to scale. For purposes of clarity, not every component is labeledin every one of the following FIGURES, nor is every component of eachembodiment of the invention shown where illustration is not considerednecessary to allow those of ordinary skill in the art to understand theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a cross section of an optical fiber,taken perpendicular to the longitudinal axis along which the fiberextends;

FIG. 2 is an idealized plot of a possible refractive index profile forthe core and cladding of the fiber of FIG. 1 as well as correspondingnormalized intensity profiles for the fundamental and selected higherorder modes (HOMs);

FIG. 3 schematically illustrates a cross section of an optical fiberaccording to the disclosure having a core and at least one region spacedfrom the core;

FIG. 4 is an idealized plot of a possible refractive index profile forthe fiber of FIG. 3 as well as corresponding normalized intensityprofiles for the fundamental and selected HOMs;

FIG. 5 shows intensity distribution maps for the fundamental and higherorder modes of another optical fiber, where only the core and claddingare considered in the analysis;

FIG. 6 shows intensity distribution maps for the fundamental and higherorder modes corresponding to the optical fiber of FIG. 5, with theproviso that only a ring core and cladding are considered in theanalysis;

FIG. 7A shows intensity distribution maps for the fundamental and higherorder modes of the optical fiber considered in FIGS. 5 and 6, where allof the core, ring core and cladding are considered in the analysis;

FIG. 7B shows intensity distribution maps for additional higher ordermodes of the optical fiber considered in FIG. 7A;

FIG. 8 shows intensity distribution maps for fundamental and higherorder modes of a additional optical fiber, with the proviso that onlythe core and cladding are considered in the analysis;

FIG. 9A shows intensity distribution maps for the fundamental and higherorder modes corresponding to the optical fiber of FIG. 8, with theproviso that only a ring core and cladding are considered in theanalysis;

FIG. 9B shows intensity distribution maps for additional higher ordermodes of the optical fiber of FIG. 9A;

FIG. 10 shows intensity distribution maps for the fundamental and higherorder modes of the optical fiber considered in FIGS. 8 and 9, where allof the core, ring core and cladding are considered in the analysis;

FIG. 11 schematically illustrates a cross section of an optical fiberaccording to the disclosure including a pair of longitudinally extendingstress inducing regions; and

FIG. 12 is a perspective view schematically illustrating one example ofan optical apparatus according to the disclosure including a secondoptical fiber located alongside a first optical fiber and a commoncladding disposed about the first and second optical fibers.

SPECIFIC DESCRIPTION

FIG. 1 schematically illustrates a cross section of an optical fiber 12,taken perpendicular to the longitudinal axis along which the fiber 12extends. The optical fiber 12 of FIG. 1 can include a core 14 and acladding 16 disposed about the core 14. Typically the cladding 16contactingly surrounds the core 12 and tends to confine optical energypropagated by the optical fiber 12 to the core 14 for guidance thereby,such as by via the phenomenon of total internal reflection. The opticalfiber 12 can include at least one additional region 18 disposed aboutthe cladding 16. The region 18 shown in FIG. 1 can represent a secondcladding disposed about the cladding 16 for tending to confine opticalenergy, such as pump optical energy, to the cladding 16 for guidancethereby. Such a fiber is referred to in the art as a “cladding pumped”or “double-clad” optical fiber, and the cladding 16 can be referred toas a “pump cladding.” The second cladding can comprise, for example, aglass or a fluorinated low index polymer coating applied and curedduring draw of the optical fiber 12. Alternatively, the region 18 canrepresent a high index polymer protective region, typically comprisingan outer higher modulus layer disposed over an inner lower moduluslayer. One or both of the layers can be applied as coatings to theoptical fiber as part of the draw process. Variations of the foregoingare of course possible. For example, the region 18 can comprise a glasssecond cladding for tending to confine pump light to the cladding 16,and the region 18 can in turn have disposed thereabout a polymerprotective region having one or more layers.

The core 14 can comprise an active material for providing optical energy(e.g., via the process of stimulated emission) responsive to the opticalfiber 12 receiving pump optical energy having a pump wavelength. Theactive material can comprise a rare earth material, such as, forexample, one or more of erbium, ytterbium, neodymium or thulium (e.g., aconcentration of Er, Yb, Nd or Th ions).

The core 14 is typically multimode, and has a diameter D that is largerthan a standard single mode core (e.g., a 5 micron diameter core) toprovide a fundamental mode having an increased mode field diameter(MFD). The increased MFD can reduce power density and hence increase thepower threshold for the onset of non-linear phenomena. The exact MFD candepend on other factors, such as refractive index profile, of course,but for many standard designs a larger diameter core will typicallyhaving a fundamental mode have a larger MFD.

FIG. 2 includes a plot of one possible refractive index profile 30 forthe core 14 and cladding 16 of the fiber 12 of FIG. 1, and also includesplots of normalized intensity profiles for the fundamental and selectedHOMs. (Intensities are normalized for each mode by setting the integralover r·dr·dφ to 1.) As shown in FIG. 2, the optical fiber 12 cansupport, in addition to the fundamental mode LP₀₁ indicated by referencenumeral 36 with a central maxima 48, multiple HOMs, such as, amongothers, the LP₁₁ mode indicated by reference numeral 38 and the LP₀₂mode, indicated by reference numeral 42. Note that whereas certain ofthe HOMs, such as the LP₁₁ mode 38 and, has a central minima or null,the LP₀₂ mode comprises an intensity profile that does not include sucha minima or null, and rather is substantially non-zero at its center, asindicated in FIG. 2 by reference numeral 46. A substantially centralmaxima (which is typically absolute, not just local) is characteristicof modes of zero azimuthal order, at least for lower order modes.Azimuthal order refers to the value of the first subscript in the modedesignation.

The HOMs, if excited, can degrade beam quality, as noted above. In manyinstances the LP₀₂ mode is the most problematic, because it shares withthe fundamental mode a intensity profile that has a substantiallycentral maxima such that excitation of the fundamental mode, such as bya simple splice to single mode fiber, could be very likely to excite theLP₀₂ mode. In other instances, the LP₁₁ mode can be problematic.

Consider now the optical fiber 412 of FIG. 3, which can be substantiallysimilar to optical fiber 12 of FIG. 1, and hence can include some or allof the structural features thereof noted above. The optical fiber 412includes a core 414 with a diameter D, a cladding 416 disposed about thecore 414, and, optionally, a second region 418 disposed about thecladding 416. However, in addition to the features of the optical fiber12 of FIG. 1, the optical fiber 412 can further include at least oneregion 423 spaced from the core 414. In the embodiment shown in FIG. 4the at least one region 423 comprises a ring core 425. The at least oneregion 423 can include an absorbing material for absorbing opticalenergy having a selected wavelength, such as, for example, thewavelength of operation of the optical fiber 412. The absorbing materialis preferably of low absorption at the pump wavelength.

FIG. 4 plots one possible refractive index profile 430 for the opticalfiber 412 of FIG. 3 and normalized intensity profiles for thefundamental and selected HOMs. The refractive index profile 430 includesan inner section 435 corresponding to the core 414 and outer sections431 corresponding to the ring core 425. Sections of the refractive indexprofile between the section 435 (corresponding to the core) and thesections 431 (representative of the ring core 425) correspond to thecladding 416, as do sections outward of the sections 431. Claddingsections are not indicated by reference numerals in FIG. 4. Note thatrefractive profile section 435 and sections 431 can have substantiallythe same index difference with respect to the cladding.

Reference numerals 436 and 438 indicate respective fundamental mode LP₀₁and HOM LP₁₁, all of which are at least relatively similar to,respectively, the LP₀₁ and LP₁₁ modes of FIG. 4 (indicated in FIG. 2 byreference numerals 36 and 38, respectively). Note that FIGS. 2 and 4differ considerably in that the intensity profile of the LP₀₂ mode ofFIG. 2, which had a substantially non-zero central portion 46 of FIG. 2,is largely absent from FIG. 4. Present are the intensity profile 448 ofthe fundamental mode LP₀₁ that is non-zero at substantially the centerof the fiber, in addition to the substantially centrally non-zerointensity profile 446 of the HOM LP₀₂, but the peak of the intensityprofile 446 is reduced considerably in comparison with the LP₀₂intensity profile 46. The LP₀₂ mode of FIG. 2 has been “converted” to amixed mode 440 of the core 414 and the at least one region 423 of FIG. 3(or in any event the LP₀₂ seems to have disappeared and the mixed modehas appeared). Because the mixed mode has appropriate intensity in thering core 425 of FIG. 3, it is substantially attenuated, including theintensity portion of the mixed mode present in the core 414, by theabsorbing material comprised by the ring core 425. The fundamental modeLP₀₁, however, which is not a mixed mode, is largely unaffected, or atleast any increase in attenuation thereof is easily accommodated and/ornot overly detrimental in many practical applications.

If the optical fiber apparatus comprises a rare earth material that isto be pumped, it can be desirable to select an absorbing material thattends not to absorb the pump optical energy. Absorbing materials usefulwith typical rare earths include samarium, praseodymium or terbium. Sm³⁺and Pr³⁺ ions, which have strong absorption around 1064 nm and 1030 nm,respectively, can be useful when the optical fiber apparatus include arare earth material, such as ytterbium, providing light at around 1060nm. Ytterbium can be pumped, for example, at 915, 940 or 975 nm, and, asone example, samarium and praseodymium have low absorption at 975 nm.Samarium can also be particularly useful when the rare earth materialcomprises neodymium or ytterbium and erbium. Terbium can be particularlyuseful when, for example, pumping the rare earth material thulium at1576 nm and samarium when pumping at 790 nm. Thulium can provide opticalenergy at about 2000 nm, as is known in the art. The absorbing materialcan have concentration of, for example, about 500 ppm, about 1000 ppm,about 1500 ppm about 2000 ppm or greater than about 2000 ppm.

The ring core can comprise a silica-based glass. The silica-based glasscan comprise, for example, one or more of a concentration of aluminum,phosphorus, germanium or fluorine. In one example, the ring corecomprises a concentration of phosphorus and fluorine, such as in asilica based glass; in another example, the ring core can comprise aconcentration of aluminum and can include, for example, a concentrationof germanium. In one example, a silica based glass can comprise thealuminum and germanium concentrations, and the absorbing material cancomprise samarium. The concentration ranges specified above for theabsorbing material can be useful for the Al, Ge, P and F materials notedabove.

Modeling indicates the attenuation of the mixed mode can be at leastbetween one and two orders of magnitude greater that than of thefundamental mode, where the attenuation is specified in terms ofdB/meter (e.g., tenths of a dB/meter for LP₀₁ compared to tens ofdB/meter for the mixed mode to which the LP₀₂ mode is converted). SeeTable I below.

TABLE I Modeled Losses of Fundamental and HOMs - Optical Fiber of FIGS.3 & 4 Modeled losses at 1060 nm (dB/m), ring core Mode doped with 1000ppm Sm LP₀₁ (fundamental) 0.1586 LP₁₁ 1.3708 LP₂₁ (converted to mixedmode) — LP(3) = (mixed core/ring core mode 446) 74.8062 LP(4) = (mixedcore/ring core mode 440) 38.0906 (Core diameter = 19 μm, Δn = 0.006,ring = 12.5/16.5 μm, Δn = 0.006)

Thus the optical fiber of FIGS. 3 and 4 can provide fiber having thebenefit of fundamental mode that has an increased MFD, which reduces thecore power density and allows higher power transmission before the onsetof non linear phenomena. The optical fiber 412 can be configured andarranged, such as by, for example, selecting one or more of thecomposition, geometrical shape, location and refractive index profilesof the core 414 and/or ring core 425, such that at the desiredwavelength of operation the optical fiber 412 supports a fundamentalmode and at least one HOM that is a mixed mode of the core and ringcore. The at least one HOM can be attenuated, while the fundamental modecan remain primarily a mode of the core 414.

Often obtaining a higher power laser or optical amplifier meansproviding a higher power of pump light to the optical fiber 412, and theoptical fiber 412 should be able to handle the high power without thefiber degrading, such as by photo darkening. As noted above in thediscussion regarding FIG. 1, the cladding 416 can comprise glass, suchas, for example, a silica-based glass. It is often desirable that thecladding 416 consist of or consist essentially of glass, such as asilica based glass or pure silica glass, to help ensure that the opticalfiber 412 can handle high power levels of light having the pumpwavelength, especially when the optical fiber 412 comprises a claddingpumped fiber. The cladding 416 can, in certain aspects of thedisclosure, be substantially homogenous and (except for the presence ofthe at least one region that effects mixed modes and, if desired, stressinducing regions) and hence not be considered as including a“microstructure” for guiding optical energy.

With reference to FIGS. 1-4 above, the modeled fiber has a core 14having a diameter D of 19 microns having a step index profile 30, withthe raised section corresponding to the refractive index of the core 14and having an index difference Δn of about 0.006 with respect to thecladding 16. Calculations were performed for an operating wavelength ofabout 1060 nm. The ring core 425 comprised a thickness T of about 4microns and is spaced from the core 414 by a spacing S of about 3microns (inner radius of ring core=12.5 microns, outer radius of ringcore=16.5 microns). The ring core 425 was considered as doped with about1000 ppm of an absorbing material (samarium). The index difference or Δnbetween the ring sections 431 (or more generally between the at leastone region) and the cladding was also considered as about 0.006, butmore generally need not be equal to the Δ between the core 425 and thecladding. The Δn between the ring section [435] and cladding can be lessthan, substantially equal to, or greater than the Δn between the coresection 425 and the cladding.

To further facilitate understanding of the disclosure, additionaldetails regarding optical fibers having one or more mixed modes as wellone or more modes that are primarily of the core or that at the leastone region are provided below. To illustrate the versatility of thedisclosure, different fiber designs are considered.

Consider a fiber generally as shown in FIGS. 3 and 4 and having a corehaving a diameter of about 21 μm and a ring core having an inner radiusof about 16 μm and an outer radius of about 21 μm. The fiber has a steprefractive profile generally as shown in FIG. 4 with a Δn for thecentral core of about 0.0025. The index of refraction of the cladding isabout 1.44968 at a wavelength of about 1060 nm. The core, consideredindividually, has a numerical aperture (NA) of about 0.085 and V-numberat 1060 nm of about 5.30.

The fiber can be analyzed as if the ring core is absent, that is,replaced by the cladding material. This is referred to herein as an“individual core” analysis and the modes as “core modes.” However,although the ring core is not present, the overlap integral between thenormalized mode intensity and the portion of the fiber the ring corewould occupy if present can be calculated. See Table II below.

TABLE II Waveguide modes in fiber with core diameter of 21.0 μm & Δn =0.0025. Mode Effective Mode overlap Mode overlap LP Mode refractiveintegral with integral mode Type index core with ring 1 LP₀₁ 1.451820.9733 0.00040 2 LP_(11o) 1.45129 — — 3 LP_(11e) 1.45129 0.9267 0.001794 LP_(21o) 1.45061 0.8529 0.00693 5 LP_(21e) 1.45061 — — 6 LP₀₂ 1.450400.8041 0.01586 7 LP_(31o) 1.44982 0.7165 0.03404 8 LP_(31e) 1.44982 — —n_(clad) = 1.44968 at 1060 nm. NA = 0.085. V= 5.30.

Modes are ordered by their effective index value, with higher ordermodes having lower effective indices. Modes having an effective indexgreater than that of the cladding are considered guided. Odd and evendegenerate modes are indicated by “o” and “e” subscripts, respectively.Only one calculation is made where values are expected to be the samefor the odd and even modes. FIG. 5 shows two dimensional intensitydistribution maps for the modes of Table II.

The optical fiber can also be analyzed as if the ring core is presentand the core is absent. Table III presents the results of such ananalysis, and FIG. 6 shows intensity maps for the modes of Table III.This analysis is referred to herein as “individual ring core” analysisand the modes as “ring core” or “ring” modes.

TABLE III Waveguide modes in fiber with ring r₁/r₂ = 16.0/21.0 μm & Δn =0.00154. Mode effective Mode overlap Mode overlap LP Mode refractiveintegral mode integral mode Type index with core with ring 1 R₀₁ 1.450410.00137 0.67097 2 R_(11o) 1.45037 — — 3 R_(11e) 1.45037 0.00065 0.673954 R_(21o) 1.45029 0.00023 0.67764 5 R_(21e) 1.45028 — — 6 R_(31o)1.45013 0.00062 0.67857 7 R_(31e) 1.45013 — — 8 R_(41o) 1.44994 0.000020.67354 9 R_(41e) 1.44993 — — 10 R_(51o) 1.44969 0.00000 0.65995 11R_(51e) 1.44968 — — n_(clad) = 1.44968 at 1060 nm.

Table IV below tabulates modeling analysis of the actual optical fiber,that is, where the central and the ring core are both present. FIG. 7shows two dimensional intensity distribution maps for the modes of theactual fiber analysis of Table IV.

TABLE IV Waveguide modes in fiber with core diameter of 21.0 μm & Δn =0.0025 and ring r₁/r₂ = 16.0/21.0 μm & Δn = 0.00154. Mode Effective Modeoverlap Mode overlap LP Mode refractive integral with integral mode Typeindex core with ring 1 LP₀₁ 1.45182 0.97237 0.00045 2 LP_(11o) 1.45129 —— 3 LP_(11e) 1.45129 0.92049 0.00343 4 LP_(21o) 1.45063 0.77994 0.048315 LP_(21e) 1.45063 — — 6 (LP₀₂-R₀₁)_(m1) 1.45051 0.38302 0.33125 7R_(11o) 1.45037 — — 8 R_(11e) 1.45037 0.01737 0.66734 9 (LP₀₂-R₀₁)_(m2)1.45026 0.47590 0.34334 10 R_(21o) 1.45025 0.00664 0.64030 11 R_(21e)1.45024 — — 12 R_(31o) 1.45016 0.06734 0.60407 13 R_(31e) 1.45016 — — 14R_(41o) 1.44995 0.00741 0.66361 15 R_(41e) 1.44993 — — 16 LP_(31o)1.44976 0.68473 0.08991 17 R_(51e) 1.44968 0.00234 0.65680 n_(clad) =1.44968 at 1060 nm.

This analysis is referred to as an “actual fiber” analysis, and themodes as “actual fiber modes” or “actual modes.”

The data presented herein in the various FIGURES and Tables is nowdiscussed to demonstrate the approach used to classify modes as mixed orprimarily of a region and to determine how to identify the modes thathave mixed to form a particular mixed mode. To better demonstrate themodal intensity distribution maps, the analysis used to generate themaps of FIGS. 5-10 does not include the effect of the absorbingmaterial, such as the absorbing material comprised by the ring 425 ofFIG. 3.

The following criteria represents one way to establish that a modeidentified in an actual fiber analysis is a mixed mode whereinparticular modes identified in individual analyses have mixed: (1) theintensity map for the actual fiber mixed mode appears to be acombination of the individual intensity maps of the individual modesthat are mixing to form the mixed mode; (2) the modes that are mixingare of the same azimuthal order; and (3) whereas the modes consideredindividually might not have intensities in the core and ring core thatare significantly of the same order of magnitude (as can be indicated bythe overlap integrations being generally the same), for the mixed modeintensities are significantly of the same order. Finally, to facilitatemixing, (4) the individual modes of the core and of the at least oneregion should have effective indices that are not too disparate. Anexact match of effective indices is not understood to be required,however.

With reference to Tables II and III, note that the effective refractiveindex of the LP₀₂ core mode (1.45040) is substantially the same as thatof the effective refractive index of the R₀₁ ring core mode (1.54041).(Note that the difference, 1×10⁻⁵, is certainly considered narrower thanthe full ambit of “substantially the same.”) Furthermore, both the modesare of the same (zero in this case) azimuthal order. In addition,whereas FIGS. 7A and 7B include intensity maps that bear a strongresemblance to each intensity maps for individual core modes orintensity maps for individual ring modes, FIGS. 7A and 7B do not includean analog to the LP₀₂ core mode. For example, the intensity maps formodes LP(1)-LP(5) of the actual fiber analysis of FIG. 7A each bear astrong resemblance to the intensity maps for the modes LP(1)-LP(5) ofthe core analysis of FIG. 5. The LP(6) mode of the actual analysis ofFIG. 7A, however, does not resemble any one core mode intensity map ofFIG. 5 or ring core mode intensity map of FIG. 6, but rather appears tobe a combination of the LP₀₂ core mode (LP(6) of the individual coreanalysis of FIG. 5) and the R₀₁ (LP(1) of the individual ring coreanalysis of FIG. 6).

The overlap integrals of Tables II-IV confirm that the LP(6) actualfiber mode is a mixed mode of the LP₀₂ core mode and the R₀₁ ring coremode. Whereas the overlap for the R₀₁ ring core mode with the ring coreis approximately 500 times the overlap with the core, and the overlap ofLP₀/core with the core is approximately 50 times the overlap with thering core, the ratio of the larger overlap value to the smaller overlapvalue for the LP(6) actual mode is now reduced to 1.1563, indicating aclose to even distribution between the core and ring core.

Based on a similar analysis for the LP(9) actual mode, it is thereforeconcluded that the LP_(N) core mode and R₀₁ ring mode mix to form twomixed modes—the LP(6) and LP(9) modes of Table IV and FIG. 7A. Inclusionof absorbing material in the ring core should suppress these mixedmodes, yet leave the LP(1), or LP₀₁, fundamental actual fiber mode farless attenuated, as it primarily a mode of the core (as are the coremodes LP(2)-LP(5), which modes include modes corresponding to the LP₁₁and LP₂₁ odd and even modes of the core).

Note that certain actual HOMs can have intensity profiles that appear tobe a mixture core and cladding modes, yet that are not considered to bemixed modes. For example, LP(10) of FIG. 7B appears to be a combinationof the LP(4), or R_(21o), ring core mode of FIG. 6 and the LP(4), orLP_(21o), core mode of FIG. 5. However, the effective refractive indicesof the R_(21o) mode (1.45029) is not that close to that of the LP_(21o)mode (1.45069), and most importantly the values for the overlapintegrals for the LP(10) actual fiber HOM differ by a factor of about100 (see Table IV), with the vast majority of the intensity overlappingwith the ring core. Accordingly, although the azimuthal order of theR_(21o) and LP_(21o) modes is the same, and both are of the oddorientation, they are not considered to be mixing to form a mixed mode,despite the LP(10) actual fiber intensity distribution appearing to be adirect combination of R_(21o) ring core mode and the LP_(21o) claddingmode. The LP(10) actual fiber mode appears to be primarily a mode of thering core, despite the added features of intensity map. A mode such atthe LP(10) is referred to herein as a “composite mode,” because of theappearance of the intensity map, but not a mixed mode.

Accordingly, one approach to mixing a selected mode of region, forexample, a selected mode of the core (perhaps so that it can besuppressed) is to design the ring core such that a ring mode of the sameazimuthal order as the targeted core mode has a similar effective indexto that of the targeted core mode. Analysis of the actual fiber data canconfirm the existence of the mixed mode. Iterations can be performed asnecessary, varying one or more of the geometry of the core and ringcores, spacing therebetween, refractive index profiles, etc. to arriveat the design where the desired modes mix to form a desired mixed mode.The effect of an absorbing material comprised in one of the regions(e.g., the core or ring core) can be ascertained to establishsuppression of a mode or to further confirm that a mode is a mixed modeor primary mode. For example, absorbing material in the ring core shouldaffect mixed modes and modes that are primarily of the ring core, buttypically do not substantially affect modes that are primarily modes ofthe core.

In the above examples, the LP₀₂ core mode is mixed with ring core modesto form an actual mode, but no attempt was made to mix the LP₁₁ coremodes. Data demonstrating an optical fiber wherein the LP₁₁ core modesmix with ring core modes to form mixed modes are presented in TablesV-VII and FIGS. 8-10. The procedure follows that described above inconjunction with Tables II-IV and FIGS. 5-7B. In this example theoptical fiber has a core having a diameter of about 14.0 μm and Δn ofabout 0.003 with respect to the cladding, which, as in the previousexample, has an index of refraction at 1060 nm of 1.44968. The core,considered alone, has a numerical aperture (NA) of about 0.093 and aV-number of about 3.87. The optical fiber includes a ring core having aninner radius of about 14 μm and an outer radius of about 20 μm, and a Δnwith respect to the cladding of about 0.00215. Again, note that the Δnof the core and ring core differ.

Table V presents the individual core analysis, and FIG. 8 shows thecorresponding intensity maps for the modes presented in Table V; TableVI presents the ring core considered individually, and FIG. 9 shows thecorresponding intensity maps for the modes of table VI; and Table VIIpresents actual fiber mode data, with FIG. 10 presenting the intensitydistribution maps for actual fiber modes of the Table VII.

TABLE V Waveguide modes in fiber with core diameter of 14.0 μm & Δn =0.003. Mode Effective Mode overlap Mode overlap LP Mode refractiveintegral with integral mode Type index core with ring 1 LP₀₁ 1.451980.94157 0.00007 2 LP_(11o) 1.45096 — — 3 LP_(11e) 1.45096 0.830740.00098 4 LP_(21o) 1.44978 0.58417 0.03368 5 LP_(21e) 1.44977 — —n_(clad) = 1.44968 at 1060 nm. NA = 0.093. V = 3.87.

TABLE VI Waveguide modes in fiber with ring r₁/r₂ = 14.0/20.0 μm & Δn =0.002l5. Mode Effective Mode overlap Mode overlap LP Mode refractiveintegral with integral mode Type index core with ring 1 R₀₁ 1.451000.00070 0.80418 2 R_(11o) 1.45096 — — 3 R_(11e) 1.45096 0.00041 0.809454 R_(21o) 1.45086 0.00019 0.81169 5 R_(21e) 1.45085 — — 6 R_(31o)1.45068 0.00007 0.81247 7 R_(31e) 1.45068 — — 8 R_(41o) 1.45044 0.000020.80934 9 R_(41e) 1.45043 — — 10 R_(51o) 1.45014 0.00001 0.79959 11R_(51e) 1.45014 — — 12 R_(61o) 1.44978 0.00000 0.78037 13 R_(61e)1.44978 — — n_(clad) = 1.44968 at 1060 nm.

TABLE VII Waveguide modes in fiber with core diameter of 14.0 μm & Δn =0.003 ring r₁/r₂ = 14.0/20.0 μm & Δn = 0.00215. Effective Mode overlapMode overlap LP Mode refractive integral with integral mode Type indexcore with ring 1 LP₀₁ 1.45198 0.94077 0.00038 2 (LP₁₁-R₁₁)_(m1o) 1.45101— — 3 (LP₁₁-R₁₁)_(m1e) 1.45101 0.39740 0.39548 4 R₀₁ 1.45100 0.002580.80793 5 (LP₁₁-R₁₁)_(m2o) 1.45091 — — 6 (LP₁₁-R₁₁)_(m2e) 1.450910.42913 0.42612 7 R_(21o) 1.45086 0.00146 0.80846 8 R_(21e) 1.45085 — —9 R_(31o) 1.45068 0.00027 0.81170 10 R_(31e) 1.45068 — — 11 R_(41o)1.45044 0.00005 0.80919 12 R_(41e) 1.45043 — — 13 R_(51o) 1.450140.00001 0.79953 14 R_(51e) 1.45014 — — 15 R_(61o) 1.44978 0.000730.77957 16 R_(61e) 1.44978 — — 17 LP_(21o) 1.44976 0.60800 0.01824n_(clad) = 1.44968 at 1060 nm.

From Tables V and VI, note the LP_(11o) and LP_(11e) core modes havesubstantially the same effective indices as the R_(11e) and R_(11o) ringcore modes. The modes, of course, are of the same azimuthal order(azimuthal order is 1 in this case). The odd modes and even modes eachmix to form two mixed actual fiber modes, resulting in a total of fourmixed modes. That is, LP_(11o) mixes with R_(11o) to form the mixedmodes LP(2) and LP(5) of Table VII and FIG. 10 (labeled (LP₁₁-R₁₁)_(m1o)and (LP₁₁-R₁₁)_(m2o), respectively). Similarly, the LP_(11c) core modemixes with the R_(11c) ring core mode to form the LP(3) and LP(6) mixedactual fiber modes indicated in Table VII and FIG. 10 (labeledLP₁₁-R₁₁)_(m1e) and (LP₁₁-R₁₁)_(m2e.), respectively). The mode fielddistribution maps of the actual fiber LP(2) and LP(5) modes appear to belogical combinations of the LP_(11o) and R_(11o) modes, and the modefield distribution maps for the LP(3) and LP(6) modes appear as onewould expect for combinations of the LP_(11e) and R_(11e) modes.Consideration of the overlap integrals also supports the formation ofthe identified mixed modes. The overlap integrals for the LP(2), LP(3),LP(5) and LP(6) actual fiber modes have ratios of the higher to lowervalues on the order of 1, indicating nearly equal distribution in thecore and ring core.

Note that the LP₂₁ core modes (i.e., the LP_(21o) and LP_(21c), coremodes) of Table V have effective refractive indices (1.44978, 1.44979)that are nearly identical those of the R₆₁ ring core modes (1.44978) ofTable VI. However, full consideration of all data presented in TablesV-VII and FIGS. 8-10 indicates that the LP₂₁ and R₆₁ modes, though“index matched,” do not mix to form a mixed mode, and remain primarilymodes of the core and ring core, respectively, in the analysis of theactual fiber. Consider also that the effective indices of the Applicantsconsider that this is because the LP₂₁ core modes are of significantlydifferent azimuthal order (order=2) than the 6^(th) order R₆₁ ring coremodes. This insight—that matching azimuthal order can greatly facilitateselecting modes for mode mixing and is, at least in some circumstances,more important than strict effective index matching and can be acondition for modes to mix—does not appear to be appreciated by theprior art.

Although examples provided herein have focused on preserving theintegrity of the fundamental mode and selective suppression of certainHOMs, the teachings herein could be applied, in certain circumstances,to favoring a selected HOM over another HOM at the expense, perhaps, ofthe fundamental mode. Such an approach is within the scope of thedisclosure. It is also considered within the scope of the presentdisclosure to have both the core LP₁₁, and LP₀₂ modes mix with ring coremodes to form mixed modes. The design may include two ring cores, onesurrounding another, where the core LP₁₁ mode mixes with a mode of onering core and the core LP₀₂ mode mixes with a mode of the other ringcore. In another approach, the core LP₀₂ can mix with the ring R₀₂ ringcore mode and the core LP₁₁ mix with a mode of the ring core having alower order than the R₀₂ mode.

Thus, according to one aspect of the disclosure, Applicant has realizedthat it may not be necessary to address all HOMs according to the sameproscription. Certain HOMs, in many applications, are much more likelyto be problematic than others, and, accordingly it may not be asimportant to address those that are less important in the same manner asthose that are more problematic. A splice from an SM fiber to a MM fiberis much more likely to excite a HOM having an intensity distribution mapthat is also substantially central and azimuthally symmetric than otherHOMs that are not substantially central and azimuthally symmetric. Forexample, such a splice is considered more likely to excite the LP₀₂ modeshown in FIG. 3 than the LP₁₁ mode.

V-number and NA are parameters that are often specified for an opticalfiber. Unless otherwise specified, V-number and NA of a core refer tothe V-number and the NA of the core considered individually, that is,without consideration of the at least one region that does contribute tothe formation of mixed modes. It is noted that the a fiber can be“microstructured,” that is, can include features, such as an array oflongitudinally extending index modified regions (e.g., an array of voidshaving an index of refraction different than that of the materialdefining the voids) that provide a photonic bandgap effect or thatmacroscopically change the average index of the cladding via a weightedaverage analysis of the indices of refraction of the silica regions andindex modified regions. In the latter instance guidance by the core isstill considered to be by total internal reflection (TIR).Microstructured fibers are considered to be within the scope of thepresent disclosure. For example, in one microstructured design, the“ring” can be formed by leaving out the voids in an annular regiondisposed about the core. In this instance, analysis of the coreindividually would include the cladding including the voids (and withthe ring including the otherwise missing voids), and a mode consideredto be guided “primarily” by the core would of course be affected by thevoids.

In another example, an optical fiber according to the present disclosurecan comprise a core, a cladding disposed about the core, and optionallya region disposed about the cladding. The optical fiber can include atleast one region spaced from the core, where the at least one region cancomprise a plurality of satellite regions, which can be individuallongitudinally extending voids or index modified regions arranged in aring or other configuration.

In certain aspects, an optical fiber according to the present disclosurecan have a core having a V-number at the wavelength of operation of thefiber of no less than 4.0; no less than 5.0; no less than 6.0; no lessthan 7.0; or no less than 7.5. In certain aspects of the disclosure theV-number can be from 3.0 to 5.0; from 5.0 to 7.0; or from 7.0 to 10.0.In other aspects of the disclosure, the V-number is not greater than 3,not greater than 3.5, not greater than 4, not greater than 4.5, notgreater than 5, or not greater than 5.5.

In other aspects of the disclosure, the core of a fiber can have an NAof no less than 0.12, no less than 0.15, no less than 0.16, or no lessthan 0.17 at the wavelength of operation of the optical fiber. The NA ofthe core can be about 0.17.

In additional aspects of the disclosure, the core of a fiber can have adiameter of at least 15 microns; at least 20 microns; at least 25microns; at least 30 microns; at least 35 microns; at least 40 microns;or at least 50 microns.

Combinations of the foregoing aspects are within the scope of thedisclosure, as is appreciated by one of ordinary skill apprised of thedisclosure herein. Additional embodiments of optical fiber are nowdescribed.

FIG. 11 schematically illustrates a cross section of an optical fiber1012 according to the disclosure that includes, in addition to a core1014 and at least one region 1023 spaced from the core 1014, a pair oflongitudinally, or axially, extending stress inducing regions, indicatedby reference numerals 1033A and 1033B. The stress inducing regions 1033Aand 1033B can help induce selected birefringence in the optical fiber,such as, for example, via the stress-optic effect. The stress inducingregions can have a thermal coefficient of expansion selected to bedifferent than that of the material of the fiber disposed about thestress inducing regions such that when fiber cools after being drawnstresses are permanently induced. Birefringence refers to at least aregion of the fiber, such as, for example, the core 1014, having asubstantially different refractive index for one polarization of lightthan for the orthogonal polarization of light. The fiber 1012 can be apolarization maintaining fiber or a polarizing fiber, depending, atleast in part, on the choice of one or more of composition, shape andlocation of the stress inducing regions. The index of refraction of thestress inducing regions 1033 can be adjusted, via the use of variousdopants, including, for example, those noted above, to be lower thanthat of the cladding 1016, substantially matched to that of the cladding1016, or even to be higher than that of the cladding 1016.

FIG. 12 shows a perspective view schematically illustrating one exampleof an optical fiber apparatus according to the disclosure. The opticalapparatus 1110 can include a first optical fiber 1112 that can include,as described above, a core 1114, a cladding 1116 disposed about the core1114, and at least one region 1123 spaced from the core 1114 forsupporting selected mixed modes with the core 1114. The optical fiber1112 can have a wavelength of operation and can include a rare earthmaterial for providing optical energy having the wavelength of operationresponsive to the optical fiber 1112 receiving pump optical energyhaving a pump wavelength. The optical fiber apparatus 1110 can include asecond optical fiber 1135 located alongside the first optical fiber, andthe second optical fiber can include at least a core 1137. The opticalfiber apparatus 1110 can include a common cladding 1145 disposed aboutthe first and second optical fibers. The common cladding 1145 can beconstructed and arranged so as to tend to confine optical energy to thecore 1137 of the second optical fiber 1135 for guidance by the core1137. The second optical fiber 1135 can propagate pump optical energyand the first and second optical fiber located alongside one another, asshown in FIG. 10, such that the pump optical energy couples to the firstoptical fiber 1112 for optically pumping the rare earth materialcomprised by, for example, the core 1114 of the first optical fiber1112. The first and second optical fibers, 1112 and 1135, respectively,can be drawn together within the common cladding 1145. The opticalapparatus 1110 can be constructed and arranged such that the first andsecond optical fibers can be accessed individually at the ends of alength of the optical fiber apparatus 1110 so as to, for example, couplepump optical energy to the second optical fiber for subsequent couplingto the first optical fiber and to deliver a signal to and/or extract asignal from the core 1114 of the first optical fiber 1112.

An optical fiber apparatus can be configured, according to one aspect ofthe disclosure, as a laser. Such as laser can comprise at least onereflector, which can comprise a grating, such as, for example, a Bragggrating formed via the selective application of actinic radiation to,for example, a photosensitive section of optical fiber. The laser cancomprise a second reflector. One of the reflectors is usually lessreflective than the other of the reflectors, as is known in the art. Twospaced reflectors can form a laser cavity therebetween. The laser canalso be configured as a distributed feedback (DFB) laser, and can use adistributed reflector, typically in the form of one grating having aphase change therein, and can provide narrow linewidth light. A lasercan also be configured in a master oscillator-power amplifier (MOPA)arrangement, where a master oscillator, such as a diode or fiber laser,seeds an optical fiber amplifier. Optical fiber apparatus according tothe disclosure can include a fiber optical coupler for coupling pumplight to the optical fiber apparatus, as well as a source of pumpoptical energy, which can comprise one or more pump diodes.

As noted above, an optical fiber can comprise a rare earth material forproviding light of a first wavelength responsive to the fiber receiving(e.g., being “pumped by”) light of a second wavelength (e.g., “pumplight”). “Rare earth material,” as used herein, means one or more rareearths, typically included in the fiber in the form of rare earth ions.The rare earths can be selected by those of ordinary skill in the art ofthe field of pumped fibers, for example from the Lanthanide group ofelements in the periodic table (materials having the atomic numbers57-71). The optical fiber can be pumped as shown in FIG. 8 and discussedabove. Also, the optical fiber can be “end-pumped” as is known in theart, and can include a second, or “pump” cladding for propagating thepump light delivered to the optical fiber via the end pumping.

The refractive index profiles shown in the foregoing FIGURES areidealized. Actual refractive index profiles measured on a preform orfrom an actual optical fiber drawn from the preform can include otherfeatures, as is well known in the art, such as rounded edges betweensections and the signature “dip” in the index of refraction of the coredue to the burnoff of dopants in the core during the collapse stage ofthe Modified Chemical Vapor Deposition (MCVD) process (assuming that theMCVD process is used to fabricate the optical fiber preform). Also, someof the sections of the refractive index profile corresponding to aparticular region of the fiber are drawn to portray the index ofrefraction as substantially constant for the region. This need not betrue in all practices of the disclosure. As is well known in the art,the index of refraction of a region of a fiber, such as the core of afiber, need not be constant, and can be varied according to apredetermined function to provide a particular result. For example, itis known in the art to provide a core comprising a graded refractiveindex profile, where the profile corresponds to a parabola or othersuitable function.

Several embodiments of the invention have been described and illustratedherein. Those of ordinary skill in the art will readily envision avariety of other means and structures for performing the functionsand/or obtain the results or advantages described herein and, each ofsuch variations or modifications is deemed to be within the scope of thepresent invention. More generally, those skilled in the art wouldreadily appreciate that all parameters, dimensions, materials andconfigurations described herein are meant to be exemplary and thatactual parameters, dimensions, materials and configurations will dependon specific applications for which the teaching of the presentdisclosure is used.

Those skilled in the art will recognize or be able to ascertain using nomore than routine experimentation many equivalents to the specificembodiments of the invention described herein. It is therefore to beunderstood that the foregoing embodiments are presented by way ofexample only and that within the scope of the appended claims andequivalents thereto, the invention may be practiced otherwise than asspecifically described. The present invention is directed to eachindividual feature, system, material and/or method described herein. Inaddition, any combination of two or more such features, systems,materials and/or methods, if such features, systems, materials and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

In the claims as well as in the specification above all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving” and the like are understood to be open-ended.Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the U.S. Patent Office Manual of PatentExamining Procedure §2111.03, 7^(th) Edition, Revision.

The phrase “A or B” as in “one of A or B” is generally meant to expressthe inclusive “or” function, meaning that all three of the possibilitiesof A, B or both A and B are included, unless the context clearlyindicates that the exclusive “or” is appropriate (i.e., A and B aremutually exclusive and cannot be present at the same time).

It is generally well accepted in patent law that “a” means “at leastone” or “one or more.” Nevertheless, there are occasionally holdings tothe contrary. For clarity, as used herein “a” and the like mean “atleast one” or “one or more.” The phrase “at least one” may at times beexplicitly used to emphasize this point. Use of the phrase “at leastone” in one claim recitation is not to be taken to mean that the absenceof such a term in another recitation (e.g., simply using “a”) is somehowmore limiting. Furthermore, later reference to the term “at least one”as in “said at least one” should not be taken to introduce additionallimitations absent express recitation of such limitations. For example,recitation that an apparatus includes “at least one widget” andsubsequent recitation that “said at least one widget is colored red”does not mean that the claim requires all widgets of an apparatus thathas more than one widget to be red. The claim shall read on an apparatushaving one or more widgets provided simply that at least one of thewidgets is colored red.

What is claimed is:
 1. Optical fiber apparatus having a wavelength ofoperation at which said optical fiber apparatus propagates opticalenergy, comprising: an optical fiber, said optical fiber including acore; a pump cladding disposed about said core, said pump cladding forreceiving pump optical energy having a pump wavelength; a secondcladding disposed about said pump cladding for tending to confine pumpoptical energy to said pump cladding for propagation thereby; said corecomprising a rare earth material for providing optical energy having thewavelength of operation responsive to said optical fiber receiving thepump optical energy having the pump wavelength; at least one ring corespaced from said core, said ring core defined by inner and outerdiameters and comprising the cross sectional area therebetween, saidring core further comprising an absorbing material for absorbing opticalenergy having the wavelength of operation; wherein said optical fiber isconfigured and arranged such that at said wavelength of operation saidoptical fiber comprises a fundamental mode that is primarily a mode ofsaid core and at least one higher order mode (HOM) that is a mixed modeof a selected mode of said core and of selected mode of said ring core,said selected modes being of the same azimuthal order; and wherein saidmixed mode is suppressed relative to said fundamental mode.
 2. Theoptical fiber apparatus of claim 1 wherein said selected mode of saidcore composes the LP₀₂ mode of said core.
 3. The optical fiber apparatusof claim 1 wherein said selected mode of said core comprises the LP₁₁mode of said core.
 4. The optical fiber apparatus of claim 1 wherein theintensity profile of said at least one HOM comprises a substantiallycentral maxima.
 5. The optical fiber apparatus of claim 1 wherein theintensity profile of said at least one HOM comprises a substantiallycentral minima.
 6. The optical fiber apparatus of claim 1 wherein saidpump cladding consists essentially of pure silica.
 7. The optical fiberapparatus of claim 1 wherein said fiber does not include amicrostructure for providing guidance of optical energy.
 8. The opticalfiber apparatus of claim 1 wherein said rare earth material comprises atleast one of ytterbium, erbium, neodymium and thulium.
 9. The opticalfiber apparatus of claim 1 wherein absorbing material comprises at leastone of samarium, praseodymium or terbium.
 10. The optical fiberapparatus of claim 1 wherein said pump wavelength comprises a wavelengthof about 790 nm and said absorbing material comprises samarium.
 11. Theoptical fiber apparatus of claim 1 wherein said pump wavelengthcomprises a wavelength of about 1567 nm and said absorbing materialcomprises terbium.
 12. The optical fiber apparatus of claim 1 whereinsaid wavelength of operation comprises a wavelength of about 1060 nm andwherein said absorbing material comprises samarium or praseodymium. 13.The optical fiber apparatus of claim 1 wherein said rare earth materialcomprises neodymium and wherein said absorbing material comprisespraseodymium.
 14. The optical fiber apparatus of claim 1 wherein said atleast one HOM is suppressed such that the propagation loss of said atleast one HOM is at least 10 times, in terms of dB per meter, higherthan the propagation loss of said fundamental mode at the wavelength ofoperation.
 15. The optical fiber apparatus of claim 1 wherein saidoptical fiber comprises at least one longitudinally extending stressinducing region having a thermal coefficient of expansion that isdifferent than material of said optical fiber disposed about said stressinducing region, said stress inducing region for increasing thebirefringence of said optical fiber for providing polarizationmaintaining propagation of the fundamental mode.
 16. The optical fiberapparatus of claim 1 wherein said optical fiber extends along alongitudinal axis and wherein said optical fiber apparatus comprises asecond longitudinally extending optical fiber, said second optical fiberlocated alongside said first optical fiber such that pump optical energypropagated by said second optical fiber can couple to said pump claddingof said optical fiber for optically pumping said active material; and aselected cladding disposed about said optical fiber and said secondoptical fiber.
 17. The optical fiber apparatus of claim 1 wherein saidoptical fiber apparatus is configured as a laser having a laser cavitydefined by at least one optical fiber Bragg orating.
 18. The opticalfiber apparatus of claim 1 wherein said optical fiber apparatus isconfigured as a laser include a master oscillator power amplifier (MOPA)arrangement.
 19. The optical fiber apparatus of claim 1 wherein saidrare earth material comprises thulium, and said absorbing materialcomprises terbium.
 20. The optical fiber apparatus of claim 1 whereinsaid ring core comprises a silica based glass comprising a concentrationof aluminum.